Fastener Grades Explained: A Technical Guide for Industrial Selection
Identifying the correct fastener grade is fundamental to ensuring the safety, integrity, and longevity of any assembly. The numbers and markings on screws, nuts, and bolts provide essential data on their mechanical properties, guiding proper selection for demanding industrial, automotive, and construction applications.
Why Fastener Grades Are Critical
Fasteners are engineered components designed to meet specific strength requirements. Using an under-specified grade risks joint failure, while an over-specified grade may be unnecessarily costly. The grading system allows engineers and procurement specialists to match the fastener's capability precisely to the application's load, stress, and environmental conditions.
| Feature | Metric | SAE |
| Marking System | Numbers (eg 8.8, 10.9) | European & Asian Vehicles, Industrial Applications |
| Strength Unit | Megapascals (MPa) | Pounds Per Square Inch (PSI) |
| Common Uses | European & Asian Vehicles, Industrial Applications |
Pounds Per Square Inch (PSI) |
Understanding the Two Primary Systems
1. ISO Metric Property Classes
Globally prevalent, this system uses a two-number code (e.g., 8.8, 10.9, 12.9).
The first number multiplied by 100 indicates the minimum tensile strength in Megapascals (MPa). (e.g., 10.9 = 1000 MPa tensile strength).
The second number represents the yield strength ratio (yield strength/tensile strength). (e.g., 10.9 = yield strength is 90% of tensile strength, or 900 MPa).
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Common Uses |
| 4.6 | 400 | 240 | Light Duty Applications, Furniture |
| 8.8 | 800 | 640 | Automotive, Machinery |
| 10.9 | 1000 | 900 | High Performance Automotive, Industrial Use |
| 12.9 | 1200 | 1080 | Critical High Strength Applications |
2. SAE Imperial Grades
Primarily used in North America, this system uses Grade designations (Grade 2, 5, 8) identified by radial line markings on the bolt head, where more lines denote higher strength. Strength values are measured in Pounds per Square Inch (PSI).
Key Comparison & Selection Guidelines
Strength Correlation: Common equivalencies in terms of strength are Grade 5 ≈ 8.8 and Grade 8 ≈ 10.9.
Material & Coatings: Fasteners are available in various materials (carbon steel, alloy steel) with protective coatings like zinc plating or hot-dip galvanizing for corrosion resistance. Note that stainless steel (e.g., A2-70, A4-80) uses a different class system, prioritizing corrosion resistance alongside guaranteed mechanical properties.
Selecting the Correct Grade: Consider:
Load & Stress: Dynamic, vibrational, or high-tensile loads require higher grades.
Material Compatibility: Prevent galvanic corrosion by choosing compatible materials/coatings for the substrates.
Environment: Outdoor, chemical, or marine settings demand appropriate corrosion protection.
| Grade | Tensile Strength (PSI) | Yield Strength (PSI) | Common Uses |
| 2 | 60000 | 50000 | Low Stress Applications, Household Use |
| 5 | 120000 | 92000 | Automotive, Construction |
| 8 | 150000 | 130000 | Heavy Machinery, High Stress Applications |
Conclusion
Understanding fastener grades is not optional but a necessity for professional assembly. Choosing the correct ISO property class or SAE grade ensures reliability, safety, and cost-effectiveness. Tianyu supplies a comprehensive range of graded fasteners, from standard 8.8 and 10.9 property classes to high-specification coatings, meeting diverse industrial demands.
Contact us now for samples: support1@tianyufasteners.com.
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